The present invention relates to a slit exposure projection device for use in copying machines, facsimile systems, image readers and the like, and more particularly to a projection device including an array of lens elements and a roof mirror.
The projection lens commonly used for copying machines or the like has the advantage of a high resolving power, large focal depth and giving a variable magnification, whereas the lens has the drawback of a large conjugate length which renders the machine expensive and large-sized. Accordingly, various compact inexpensive projection devices have been proposed which comprise an array of small lenses.
These proposals include, for example, an arrangement wherein a plurality of small lenses are arranged on a common optical axis as a set, and a plurality of such sets are aligned to provide an array of small lenses. With such a small lens array, however, many small lenses need to be arranged on the optical axis in view of the aberration characteristics, while to assure the desired brightness, there is a need to use two or three arrays, which result in an increased cost. The arrangement then fails to fully shorten the distance between the object plane and the image plane.
It is also proposed to use an array of light transmission fibers of graded refractive indexes. The light transmission fiber is given by ion exchange a distribution of refractive indexes radially thereof, and the array comprises such fibers arranged side by side in a straight row. Although providing a shortened conjugate length between the object plane and the image plane, the array has the drawback of a great loss of quantity of light, small focal depth and high cost.
Another projection device is known which comprises an array of lenses and an array of roof mirrors disposed in the rear of the lens array separately therefrom and each having roof mirror faces for the corresponding lens. However, it is difficult to make the entire projection device uniform in image forming characteristics afforded by the combination of each lens and the corresponding mirror faces. The device has another drawback in that it is also difficult to accurately position the roof mirror array for the lens array.